Internal combustion engines may be provided with sensors for monitoring exhaust gases. Generally, sensors are provided with a sensing element located within a sensor housing intended to be exposed to exhaust gases. Typically, sensors are arranged at the exhaust pipes of an exhaust system by screwing the end portion of the sensor in a threaded bore provided on the exhaust pipe. As a result, the sensing element is exposed to exhaust gases so that sensors can collect exhaust gas data such as temperature, pressure, chemical composition, etc. of the exhaust gases.
Exhaust gas data can be useful for controlling emissions and to improve the efficiency of the engine. For example, sensors like Lambda sensors or oxygen (O2) sensors can be used for closed looped control of the fuel quantity to be injected by injectors to obtain an optimal air/fuel ratio. Such sensors are provided with a sensing element (typically a ceramic element) for detecting the residual oxygen in the exhaust gases. In this way, a precise indication of whether combustion is complete or not is given by the exhaust gas sensor, allowing to improve the efficiency of combustion and to reduce the amounts of both unburnt fuel and nitrogen oxides (NOx) entering the atmosphere. In other words, by monitoring the composition of exhaust gases (e.g. by detecting the residual oxygen in exhaust gases), it is possible to determine if the air/fuel mixture during the combustion was rich or lean and based on this information the air/fuel ratio can be adjusted to obtain an optimal value (in most cases close to stoichiometric).
Normally, the sensing element is arranged in a sensor housing provided with a protective cap (or protective tube) for providing a mechanical protection for the sensing element during shipping. The protective cap is also provided with openings designed to allow gas contact with the sensing element.
During the operation of an internal combustion engine, exhaust products (soot, unburned hydrocarbons, ashes, etc.) are generated which can flow through the exhaust pipe line. As a result, exhaust gas deposit can accumulate so as to partially, or completely, covering the protective cap and/or obstruct the related openings. In this condition the functionality of the exhaust gas sensor (accuracy, response time) is limited or null. Replacement or maintenance of the exhaust gas sensor is thus needed, with consequent waste of time and money.
Furthermore, some exhaust gas sensors (e.g. Lambda sensors) are provided with a sensing element that must be heated up to an operative temperature. In these sensors, a heater is integrated in the sensing element. Thus, after an engine cold start, the sensing element is ready to operate after a time period called “light-off time” during which the sensing element reaches the operating temperature. In some cases, the light-off time can be very long due to the presence of condensed water inside the protective cap. In particular, in order to avoid the contact of condensed water with the hot sensing element, which may damage the sensor and lead to failure, the sensing element is heated slowly, or it is not heated, when the engine is cold. The sensing element reaches the operative temperature and starts its operation only when exhaust gases reach the end of the dew point, so that condensed water is completed removed from the exhaust gas.
During the light-off time, signals coming from exhaust gas sensors cannot be used for regulating the engine operating values (e.g. the air/fuel ratio) and thus only an open loop control can be provided based on predetermined values stored in a map. In this situation, emissions and efficiency of the engine are not optimal.